Generally, the process of electrostatographic copying is initiated by exposing a light image of an original document to a substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges its surface in areas which correspond to non-image areas in the original document while maintaining the charge in image areas. This selective discharging scheme results in the creation of an electrostatic latent image of the original document on the surface of the photoreceptive member. This latent image is subsequently developed into a visible image by a process in which developer material is deposited onto the surface of the photoreceptive member. Typically, this developer material comprises carrier granules having toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically attracted from the carrier granules to the latent image for forming a powder toner image on the photoreceptive member.
Alternatively, liquid developer materials comprising a liquid carrier material having toner particles dispersed therein have been utilized. In a process such as this, the developer material is applied to the latent image with the toner particles being attracted toward the image areas to form a liquid image. Regardless of the type of developer material employed, the toner particles of the developed image are subsequently transferred from the photoreceptive member to a copy sheet, either directly or by way of an intermediate transfer member. Once on the copy sheet, the image may be permanently affixed to provide a "hard copy" reproduction of the original document or file. The photoreceptive member is then cleaned to remove any charge and/or residual developing material from its surface in preparation for subsequent imaging cycles.
The above described electrostatographic reproduction process is well known and is useful for light lens copying from an original, as well as for printing applications involving electronically generated or stored originals. Analogous processes also exist in other printing applications such as, for example, digital laser printing where a latent image is formed on the photoconductive surface via a modulated laser beam, or ionographic printing and reproduction where charge is deposited on a charge retentive surface in response to electronically generated or stored images. Some of these printing processes develop toner on the discharged area, known as DAD, or "write black" systems, in contradistinction to the light lens generated image systems which develop toner on the charged areas, knows as CAD, or "write white" systems. The subject invention applies to both such systems.
When using liquid toners, there is a need to remove the liquid carrier medium from the photoconductive surface after the toner has been applied thereto. This prevents the liquid carrier from being transferred from the photoreceptor to the paper or to the intermediate medium during image transfer. Removing the liquid carrier also allows it to be recovered for recycle and reuse in the developer system. This provides for additional cost savings in terms of printing supplies, and helps eliminate environmental and health concerns which result from disposal of excess liquid carrier medium.
One known method of removing excess carrier fluid from a developed image requires placing a blotter roll in rotatable contact with the image while it resides on the photoreceptor or intermediate substrate. The blotter roll will typically be made from an absorbent material, which allows the excess carrier fluid to be drawn from the surface of the belt and into the contacting roll. The fluid is then removed from the roll via a vacuum applied to the interior cavity of the roll. Removal of carrier fluid from the surface of the image results in an increase in solid particle content, thereby allowing for greater efficiency of the process of transferring the image from the photoreceptor to the intermediate substrate or from the intermediate substrate to permanent media. The solid content of the toner particles can be increased even further if a High Solids Image Conditioning (HSIC) unit (which includes a high pressure blotter roll) is used.
The most efficient conditioning of an image to increase the percentage of solids residing therein obviously requires preventing the solid toner particles from leaving the image while removing carrier liquid. Successful image conditioning also requires electrostatic forces to hold or stabilize the toner particles in order to increase the clarity and resolution of the toner image. In addition, the carrier liquid removal device must also remain clean and free of toner particles so as to prevent it from thereafter contaminating a subsequent image with embedded toner particles.
Various techniques and devices have been devised for conditioning the liquid developer image by using blotter rolls or rollers to remove carrier liquid from the image as discussed above. Using one method, the developed image containing approximately 8% to 1 0% solid particles is first subjected to treatment by a Low Solids Image Conditioner (LSIC) which increases the percentage of solids to approximately 14% to 20%, while increasing the stability of the image, and reducing the thickness of the background fluid. High Solids Image Conditioning (HSIC) is then applied in order to increase the solid particle content to approximately 40%-45%, enabling the image to be transferred and fixed to a final substrate, without removing solid particles along with the carrier fluid. While applying high pressure has been quite effective in increasing the solid particle content, it unfortunately also results in offset of a substantial amount of the image to the blotter surface when the input image reaches higher toner concentrations. Thus, it is advantageous to devise a way in which the solid particle content of an image developed using a liquid material may be substantially increased without requiring a high pressure to be applied to the surface of the image. In addition, it is advantageous to remove the fluid from the blotter roll by applying a vacuum to it externally rather than internally because elimination of the hollow interior cavity increases the strength of the roll and decreases the costs of manufacturing it. Applying the vacuum externally also helps to keep the conditioning roll clean.
The following disclosures may be relevant to various aspects of the present invention:
U.S. Pat. N. 5,481,341 to Sypula et al., issued Jan. 2, 1996, discloses a roller for controlling the application of carrier liquid to an image bearing member in an electrostatographic reproduction apparatus having a rigid porous electroconductive supportive core, a conformable microporous covering provided around the core, and a pressure controller. The pressure controller is located to provide a positive or negative pressure within the porous core and across a cross section of the core and covering.
U.S. Pat. No. 5,424,813 to Schlueter et al., issued Jun. 13, 1995, and having a common assignee as the present application discloses a roller comprising an absorption material and a covering, which are adapted to absorb carrier liquid from a liquid developer image. The covering has a smooth surface with a plurality of perforations, to permit carrier liquid to pass through to the absorption material at an increased rate, while maintaining a covering having a smooth surface which is substantially impervious to toner particles yet pervious to carrier liquid so as to inhibit toner particles from departing the image.
U.S. Pat. No. 5,332,642 to Sims et al., issued Jul. 26, 1994, having a common assignee as the present application, discloses a porous roller for increasing the solids content of an image formed from a liquid developer. The liquid dispersant absorbed through the roller is vacuumed out through a central cavity of the roller. The roller core and/or the absorbent material formed around the core may be biased with the same charge as the toner so that the toner is repelled from the roller while the dispersant is absorbed.
U.S. Pat. No. 4,286,039 to Landa et al., issued Aug. 25, 1981, discloses an image forming apparatus comprising a deformable polyurethane roller, which may be a squeegee roller or blotting roller which is biased by a potential having a sign the same as the sign of the charged toner particles in a liquid developer. The bias on the polyurethane roller is such that it prevents streaking, smearing, tailing or distortion of the developed electrostatic image and removes much of the carrier liquid of the liquid developer from the surface of the photoconductor.
All of the references cited herein are incorporated by reference for their teachings.
Accordingly, although known apparatus and processes are suitable for their intended purposes, a need remains for alternative methods to condition images that have been developed by liquid developer material to increase their solid content before transfer to an output copy sheet.